Broad-band guided-wave electrooptic modulators

Lumped-element Mach-Zehnder interferometric modulators have been designed, fabricated in LiNbO3, tested and analyzed. These modulators had 3 dB bandwidths from 280 MHz to 2.75 GHz and Vπ's from 1 V to 4 V, respectively. A simpleRLCequivalent circuit is utilized to model the packaged modulator performance and results are compared with those measured using a swept-frequency technique. The model is seen to break down at the higher frequencies due to a frequency-dependent resistance and the electrode and parasitic inductances are seen to limit the overall modulator performance. The effects of varying the electrode capacitance, resistance, or inductance on modulator performance are demonstrated.     

High-frequency electrooptic Fabry-Perot modulators

Electrooptic modulators built from GaAs/Al/sub x/Ga/sub 1-x/As Fabry-Perot cavities operating up to 6.5 GHz are reported. The measured frequency response agrees well with the one predicted using an equivalent circuit model derived from high-speed electrical measurements. The parasitic capacitances have been reduced to approximately 30 fF by fabricating the devices on semi-insulating GaAs substrates and integrating them with on-wafer bound pads which have dimensions compatible with microwave coplanar probes.<>     

Multigigahertz lumped-element electrooptic modulators

Multigigahertz-bandwidth lumped-element optical-guided-wave Mach-Zehnder interferometric modulators have been designed, fabricated in LiNbO3, tested, and analyzed. Three modulators were built having 3 dB bandwidths of 2.75, 4.4, and 7.3 GHz with Vπ's of 7, 14, and 28 V, respectively. A simple RLC equivalent circuit model adequately predicts the packaged modulator performance. Coupled with previously reported work, this demonstrates the ability to reliably design and fabricate lumped-element modulators which operate from dc through 7.3 GHz.     

Broad-band linearization of a Mach-Zehnder electrooptic modulator

Analog optical-link dynamic range in excess of 75 dB in a 1-MHz band has been achieved using specially designed electrooptic modulators that minimize one or more orders of harmonic and intermodulation distortion. To date, however, such “linearized” modulators have only enabled improved link dynamic ranges at frequencies below 1 GHz. Additionally, linearization across more than an octave bandwidth has required precise balancing of the signal voltage levels on multiple electrodes in a custom modulator, which represents a significant implementation challenge. In this paper, a link linearization technique that uses a standard Mach-Zehnder lithium-niobate modulator with only one RF and one dc-bias electrode to achieve broad-band linearization is discussed, resulting in a dynamic range of 74 dB in 1 MHz across greater than an octave bandwidth (800-2500 MHz). Instead of balancing the voltages on two RF electrodes, the modulator in this new link architecture simultaneously modulates optical carriers at two wavelengths, and it is the ratio of these optical carrier powers that is adjusted for optimum distortion canceling. The paper concludes by describing a second analogous link architecture in which it is the ratio of optical power at two modulated polarizations that is adjusted in order to achieve broad-band linearization     

Bandwidth of linearized electrooptic modulators

Many schemes have been proposed to make high dynamic range analog radio frequency (RF) photonic links by linearizing the transfer function of the link's modulator. This paper studies the degrading effects of finite transit time and optical and electrical velocity dispersion on such linearization schemes. It further demonstrates that much of the lost dynamic range in some modulators may be regained by segmenting and rephasing the RF transmission line     

Substrate removed GaAs-AlGaAs electrooptic modulators

A novel GaAs-AlGaAs electrooptic modulator utilizing push-pull metal electrodes was fabricated using substrate removal techniques and polymer integration. The scheme enables a significant drive voltage reduction over previous designs and should enable ultrahigh-speed adjustable chirp operation     

Analysis of circular-polarization electrooptic amplitude modulators

An analysis of electrooptic (EO) amplitude/IM modulators of light having the form of circular polarizer-sample-circular analyzer (CPSA) systems is presented. On the assumption of nonideal components, with the sample being a linear EO crystal, principal characteristics and performance parameters of the above modulators are derived and investigated as to the effect of azimuthal misalignments &thetas; in the crystal birefringence axes. The so obtained results are compared with those drawn from the study of linear-polarization EO amplitude modulators structured as linear polarizer-sample-linear analyzer (LPSA) systems in their optimized variety (45°-CLPSA modulators). From this comparison, it is found that a CPSA modulator in its optimized configuration (ACPSA modulator) is superior in EO-modulation sensitivity and considerably more stable against misalignments &thetas; than its 45°-CLPSA counterpart. On the other hand, both kinds of modulator exhibit identical behavior as regards harmonic distortion and maximum-nonlinearity parameters     

Proposed wide-band and low-drive-power electrooptic modulators

Novel methods for the wide-band and low-drive-power electrooptic modulators are proposed. This wide-band and low-drive-power performance is obtained by the utilization of relative electrical and optical delays. Both a lumped and a traveling-wave configuration are described.     

40-GHz polymer electrooptic phase modulators

A broad-band high-frequency traveling wave electrooptic phase modulator made from stable nonlinear optical polymers has been tested. Using a 1.319-μm light source, single-mode operation was achieved with high-input optical power, resulting in improved modulation depths and signal-to-noise ratios. The high-frequency optical modulation was observed up to 40 GHz using external microwave mixers. Direct optical-heterodyne detection of electrooptic phase modulation has also been demonstrated with high sensitivity at 40 GHz. The devices were tested with 10⁵ W/cm² power density for 21 hours, and have showed no observable decay in nonlinearity     

Acoustooptic resonance in deep-etched GaAs-AlGaAs electrooptic modulators

Deep-etched GaAs-AlGaAs waveguide modulators are shown to generate acoustic waves that can cause elastooptic anomalies in the modulator response. The theory of acoustooptic interactions in linear electrooptic modulators is described and a one-dimensional (1-D) approximation of a deep-etched GaAs-AlGaAs waveguide modulator is developed. The 1-D theory is shown to predict the acoustooptic resonance phenomena that is seen in experimental devices.     

Synchronization bandwidth of capacitive transverse-field electrooptic modulators

Expressions are given that can be used to calculate the synchronization bandwidth of center-fed and end-fed, capacitive electrooptic modulators that employ a modulation field transverse to the direction of optical beam propagation.     

Measurement of velocity mismatch in traveling-wave electrooptic modulators

For traveling-wave electrooptic modulators, we describe an experimental method to determine the sign and magnitude of velocity mismatch between electrical and optical wave. To this end the waves are counterpropagated and frequencies yielding modulation nulls are determined. Such a measurement does also reveal dispersion of the modulator structure.     

Quasi-static analysis of electrooptic modulators by the method oflines

A quasi-TEM analysis using the method of lines is implemented to calculate the electric field anywhere within a layered guided wave structure at appropriately low frequencies. The analysis provides the electric fields and transmission-line parameters for a shielded structure having any number of coplanar electrodes within an arbitrary number of anisotropic dielectric layers, the principal axes of which are parallel to the axes of the structure. The practical application considered is the calculation of the phase shift of a guided optical wave within an electrooptic modulator that has a dielectric buffer layer between the electrodes and electrooptic substrate     

Traveling-wave ring circuit for resonant enhancement of electrooptic modulators

Traditional implementations of resonantly enhanced electrooptic modulators are limited in their modulation enhancement and tuning ability. In this letter, a novel electrode structure based on a traveling-wave resonant ring circuit is described for which resonant frequency and resonator quality factor are easily tunable. Experimental results are presented which demonstrate significant improvements over traditional standing-wave designs.     

Digital and quasi-linear electrooptic modulators synthesized from directional couplers

By synthesizing from cascaded directional couplers, we propose new designs to achieve digital and quasi-linear electrooptic modulations. For the digital modulator, a nearly perfect on-state or off-state can be obtained in a broad range of modulation voltages. The calculated switching voltage can be as low as the drive voltage of a conventional directional coupler. The quasi-linear modulator is achieved by adding a set of compensating electrodes to a foreshortened directional coupler. Deviation from a linear voltage-intensity relationship of less than 1 percent can be obtained over a 93.5 percent modulation depth. The calculated second harmonic and the third order intermodulation distortions are -18.3 and -32.6 dB, respectively.     

Trading coupling loss for modulation efficiency in electrooptic modulators

Phase modulators utilizing the electrooptic effect in Li-NbO3are most efficient if the waveguide, and hence the guided mode, is positioned as closely as possible to the modulation electrode which is located at the surface of the crystal. However, placing the guided wave near the crystal surface tends to deform its shape so that the coupling loss between the waveguide mode and an optical fiber is increased. In this paper, we study the tradeoffs between coupling loss and modulation efficiency that result from moving the waveguide further below the crystal surface.     

High field poling of electrooptic etalon modulators on CMOSintegrated circuits

Reflection etalon modulators with electrooptic polymer layers as the spacer have been fabricated on the surface of a CMOS chip and successfully poled under a dc electric field at 80 V/μm. This is the first demonstration that such etalons can be fabricated on finished VLSI circuits and poled successfully without damage to the electronics while connected to the underlying circuitry. Such modulators, fabricated on the surface of integrated circuits, have potential applications in areas such as optical communication and free-space interconnects for multiprocessors     

Monolithic integration of waveguide polymer electrooptic modulators on VLSI circuitry

We demonstrated some of the critical technology that is needed for the monolithic integration of polymer electrooptic modulators and VLSI circuitry by fabricating and testing a phase slab modulator on nonplanar VLSI circuits. We demonstrated the survival of GaAs MESFET's to the high-voltage poling and polymer modulator fabrication procedures. We also implemented an electrical interconnect scheme between the electronics and photonics layers.     

Transmission line aspects of the design of broad-band electrooptic traveling-wave modulators

This paper proposes a two-layer traveling-wave type electrooptic modulator structure to maximize modulation bandwidth from transmission line aspects. This structure can be designed to satisfy the velocity matching and impedance matching condition simultaneously. A design method for this structure is discussed.